The present invention relates generally to nuclear :fuel assemblies for use in nuclear reactors. More specifically, the present invention relates to a coolant vent duct for use in light water reactors, and more particularly to either a boiling water reactor or a pressurized water reactor. The coolant vent duct could be applied in conjunction with a part length fuel rod, with a water rod, with a water/fuel rod or simply by itself. In addition, the present invention relates to a part length fuel rod having an upper end fitting which functions to separate at least a portion of the liquid and the vapor portions of two phase flow.
It is known to generate large amounts of heat and energy through nuclear fission in a nuclear reactor. Energy is dissipated as heat in elongated nuclear fuel rods. Typically, a nuclear fuel assembly includes a number of nuclear fuel rods that are grouped together to form a nuclear fuel assembly. Such fuel assemblies include a number of elongated rods supported between upper and lower tie plates.
It is known in boiling water reactor (BWR) fuel designs to include within fuel assemblies part-length fuel rods. Accordingly, some of the fuel rods in a bundle are truncated at some intermediate elevation in the core. This leaves an unfilled coolant channel above that elevation. By providing a truncated fuel rod, several important benefits are achieved. For example, there is a neutronic advantage in increasing the amount of fuel in the bottom of the core as compared to the top of the core. A more axial uniformity in water to fuel ratio is thereby achieved with an associated improvement in fuel cycle costs, increased shut-down margin, reduced pressure drop (principally because of increased flow area, but decreased wetted surface also reduces the pressure drop), and increased core stability because the pressure drop reduction occurs at the top part of the bundle where two phase pressure drops are most significant.
Potentially, the part-length fuel rod could yield a small critical heat flux (CHF) benefit because of the reduced mass flux in the top part of the bundle. This potential is generally not achieved. An important factor is considered to be that the simple truncation of the part-length fuel rod results in large open subchannels that have less power density than the other subchannels in the bundle. This results in significant non-uniformities of subchannel enthalpy rises.
Effectively, the flow in the other regular subchannels is reduced by a factor greater than one would expect merely from the increase in the bundle flow area that occurs above the top end of a part length fuel rod. Mixing devices and flow strippers have been utilized in an attempt to offset this problem somewhat at the expense of added pressure drop.
A number of part length fuel rod constructions have been utilized in the prior art.
U.S. Pat. No. 4,664,882 discloses a segmented fuel and moderator rod and fuel assembly for a boiling water reactor. The segmented rod has a lower fuel region and an upper moderator region for passing coolant through the upper portion of the boiling water reactor core which is normally undermoderated. The segmented rod displaces one or more conventional fuel rods in the fuel bundle.
U.S. Pat. No. 2,998,367 discloses a core that includes short fuel rods, rods of intermediate length, and rods extending the full height of the core, immersed in light water.
U.S. Pat. No. 4,789,520 discloses in an embodiment a fuel assembly having six fuel rods, each having a short fuel effective length portion in comparison to other fuel rods that are included in order to reduce the pressure loss within the fuel assembly.
U.S. Pat. No. 4,957,698 discloses a fuel design that preferentially directs more unvoided water coolant into the upper region of the fuel assembly. This allows relatively more fuel to be placed in the lower portion of the fuel assembly. The arrangement is designed to allow moderation of neutrons in the upper portion of the assembly while preserving a higher volume of fuel in the lower portion. The larger number of fuel rods that can be used in the lower portion reduces the linear heat generation (power peaking) in the assembly.
U.S. patent application Ser. No. 07/737,859, filed on Jul. 30, 1991, entitled: "IMPROVED FUEL ASSEMBLY FOR BOILING WATER REACTORS", and assigned to the assignee of this patent application discloses, in part, a fuel assembly.